The long-term objectives of this project are to examine the changes in cochlear nonlinearity in aging individuals and relate these changes to auditory perception. Specifically, the aims of the project seek to (1) determine whether aging produces a more linear cochlear response as measured by otoacoustic emissions (OAE) and (2) determine the effect of age on perceptual loudness growth and relate these measures to distortion product otoacoustic emissions (DPOAE). Past work has established that aging has a greater effect on the nonlinear distortion emissions (DPOAE) than other emissions that do not require cochlear nonlinearity, such as the linear reflection-type emissions. In a healthy cochlea, the DPOAE grows in a near-linear fashion as a function of level for low-level signals, then growth becomes compressive at moderate intensities. DPOAE input/output (I/O) functions provide a remote estimate of the compressive characteristics of the cochlea. In addition, there is evidence that the perception of loudness, while having central factors, is related peripherally to the mechanical nonlinearities of the cochlea. The experiments outlined in this project will allow for the comparison of the DPOAE level-dependent growth, the magnitudes of the different emission types, as well as perceptual loudness growth across three different age groups (young-, middle-, and older-aged). The first aim of determining whether the cochlea generates a more linear response with age will be addressed by (a) measuring the DPOAE as a function of level to provide an index of cochlear compression and (b) comparing the magnitude of the distortion emissions, which are indicators of cochlear nonlinearity, versus reflection emissions (stimulus frequency OAEs), which are generated by a linear process. For aim 1a, the linear and compressive features of the DPOAE input/output functions of young, healthy ears as well as of aging ears with and without hearing loss will be examined to investigate whether the cochlea becomes more linear as it ages, independent of hearing loss. Aim 1b seeks to measure whether aging preferentially reduces the magnitude of the nonlinear distortion emission relative to the linear reflection emission. The second aim, involving examination of the effect of aging on perceptual loudness growth, will be accomplished by conducting a measure of categorical loudness scaling with all the participants and comparing the results across the three different age groups. Furthermore, this experiment will examine the relationship between changes in the perception of loudness and the physiological changes measured in aim 1a to determine whether the physiologic changes in cochlear nonlinearity can account for changes in the perception of loudness. This work is relevant to hearing health field because it will provide information regarding the changes in the aging auditory peripheral system. By understanding age-related changes in cochlear function and its impact on auditory perception, intervention and remediation may be more targeted and effective.